The invention relates to a drive unit for a sewing machine, which may be utilized in an industrial sewing machine or embroidery machine.
A prior art unit of this kind is disclosed, for example, in Japanese Patent Publication No. 44,351/1989.
A drive unit for a sewing machine generally comprises an upper shaft for driving a sewing needle for an elevating motion, and a lower shaft for driving a shuttle race in which a lower thread is loaded for rotation. Since it is necessary that the upper and the lower shaft be driven in exact synchronism, an electric motor, serving as a drive source, is connected to the upper shaft, and a connecting rod having a gear mounted thereon is disposed vertically so that gears on the connecting rod are effective to connect the upper and the lower shaft together, as disclosed in Japanese Patent Publication No. 44,351/1989.
When using a drive unit thus constructed, it is necessary to dispose the connecting rod at a location removed from a sewing needle or the like in order to avoid an interference of the connecting rod with a movement of a fabric being sewn or an embroidery frame. However, because the size of the entire sewing machine cannot be allowed to increase freely, the distance between the location of the connecting rod and the sewing needle is not so large in actuality. Accordingly, the fabric being sewn or the embroidery frame can be moved in a range which is limited by the location of the connecting rod, thus preventing a fabric of a larger size from being sewn.
The presence of the connecting rod also prevents the location of the upper and the lower shaft from being displaced. In other words, the vertical distance between the upper and the lower shaft cannot be changed, and hence the height of an article such as an embroidery frame which must be moved into and out of the space defined between the sewing needle and a throat plate is subject to a limitation by the length of the connecting rod, which means that an article having an increased height cannot be used.
To accommodate for this, Japanese Patent Publication No. 44,351/1989 utilizes independent electric motors for driving the upper and the lower shaft, respectively, thus dispensing with the connecting rod. The elimination of the connection between the upper and the lower shaft removes any interference with the movement of a fabric being sewn, and accordingly, a sewing machine of a small size is capable of sewing a large fabric.
However, the upper and the lower shaft must be driven in synchronism with each other, and accordingly when they are driven by independent motors, a complex control circuit is required in order to assure the synchronism, resulting in an increased cost of the sewing machine.